Ya I had a plug pop on me on my 99 expedition also, what a pain in the ass

Mine was due to a shop that changed the plugs, and didnt put one in right. I had the heads pulled, bought a used head, and did a slight valve job at Dave's shop. What I have found out is that on the 4.6, 5.4 V8 (mine) and the 6.8 V10 engines from 1999 to 2004, whether in trucks or the Mustang or Crown Vic, used a spark plug with only 4 threads securing it to the aluminum head. Even plugs that have never been changed or removed, since it was built at the factory, have blown. When they blow, can damage things, like the coil, (mine

) fuel rail, or maybe even the hood. Helicoils, fail early, and if they dont, the stainless insert doesnt conduct as much heat to the head from the plug as bare aluminum, making the plug run hot. From what I read, Jasper engines, automatically put in ALL new inserts in everything they are building. The new inserts have 8 threads, instead of the 4. Also read that repair kits are available at CalVan, time-sert, and lock-N-Stitch. JMO